You can use phpseclib, a pure PHP RSA implementation:
<?php
include('Crypt/RSA.php');
$privatekey = file_get_contents('private.key');
$rsa = new Crypt_RSA();
$rsa->loadKey($privatekey);
$plaintext = new Math_BigInteger('aaaaaa');
echo $rsa->_exponentiate($plaintext)->toBytes();
?>
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